Hopefully you are successful, but how can you scale? If it takes 5 seconds on a desktop, then a server can solve 500.000 captchas per month. At $5 per month, a spammer can still send 1.000 messages for a cent.
With that the cost (and complexity) of an attack can hopefully be in the same ballpark (or higher) than ReCaptcha - without your end user having to label cars or send data to Google.
But in the end a determined spammer will get through any captcha cheaply (for reference: ReCaptcha solves are sold by the thousands for $1) - we just hope we can do better than ReCAPTCHA, especially UX-wise.
Maybe the difficulty could scale as a property of how similar the IP address is to previously seen addresses... so the addresses in the same /64 block would be very closely related, for example. (I think that's how IPv6 works... but definitely something I haven't researched lately, so I could just sound very confused)
I would personally treat any /64 as the same. Depending on your local network setup the second half of the address could be anything and could change frequently. You might also get multiple addresses. Whereas getting a new /64, or /48, requires slightly more effort.
Of course there's a risk you'll block a /64 and that takes out some whole company or whatever, but I've seen that happen to corporate proxies that got flagged as a source of spam as well so this is not an easy problem even without the 2^128 address space.
[1] Some examples: https://rhinosecuritylabs.com/aws/bypassing-ip-based-blockin... https://oxylabs.io/products/real-time-crawler https://github.com/alex-miller-0/Tor_Crawler https://www.scrapinghub.com/crawlera/